A comparison of enzymes involved in xenobiotic biotransformation was carried out in two fish species, the invasive Solea senegalensis and the native S. solea, to study their adaptability to the environmental contamination by organophosphorous pesticides in Ebro Delta Region. To this end, juveniles male and female of both species were sampled in February 2011 from the Ebro Delta region; and selected enzymes acetylcholinesterase (AChE), carboxylesterase (CbE), ethoxyresorufin O-deethylase (EROD) and glutathione S-transferase (GST) were determined in specific tissues (muscle, gonads, liver and kidney).